In U.S. Pat. No. 2,323,554 of Richard D. Mason, a vacuum cleaning apparatus is disclosed wherein a rotor actuated switch and timing means are provided to interrupt the operating circuit of the driving motor of the cleaner a predetermined time interval after movement of the nozzle over the surface undergoing cleaning is arrested. The time interval is made to be long enough to permit stopping of movement of the vacuum cleaner for short intervals without interruption of the operating circuit.
In U.S. Pat. No. 2,451,816 of George B. Dunn, a web break detector utilizes a photoelectric means responding to indicia printed on the web to operate a control to actuate an alarm in response to a breakage of the web. The alarm circuit is arranged to give a visual or audible alarm, as desired, or to automatically disconnect the driving motor of the machine.
Arthur G. Corkran, in U.S. Pat. No. 2,467,582, shows a speed responsive motor control system utilizing a control which is responsive to rotary speed of a device, such as for shutting down the drive motor thereof when the motor speed is reduced to a predetermined number of revolutions per minute. The circuit utilizes an electron discharge tube having a grid on which is impressed a periodic output potential from a transformer so as to render the tube conducting as long as the periodically varied magnetic coupling between the primary and secondary windings of the transformer is sufficiently frequent. The circuit further includes means to delay the dropout of the relay to provide the maintained energization as long as the frequency is at the desired value.
In U.S. Pat. No. 2,472,526, Albert C. Frazee shows an underspeed motor protective system for use in connection with a motor driven conveyor belt. The control utilizes a cam for maintaining energization of a relay.
Robert D. Miller et al, in U.S. Pat. No. 3,389,321, show a frequency sensing detector for stopping a motor upon slowdown. The control includes means to generate electrical pulses at a frequency proportional to the advancement rate of an apparatus with means for receiving the pulses to effect the energization of the motor in response to a predetermined decrease in the frequency of pulse reception.
In U.S. Pat. No. 3,536,977, Virgle E. Porter shows a control circuit for stopping a motor in response to a torque overload. The control circuit senses a preselected drop in the speed of a device by means of a signal produced in each cycle of rotation thereof. If the signal is of both an appropriate duration and amplitude, the control apparatus is actuated. The control includes a rotating disc passing between a stationary magnet and a reed switch so as to open the switch once during each revolution of the disc. The opening and closing of the switch provides a pulse with the frequency of the pulse generation being sensed in the control circuit for controlling the operation of the motor.
James W. Grygera discloses, in U.S. Pat. No. 3,728,604, a motor control system wherein the electromotive force generated by the motor and the motor armature current are sensed on starting and stopping. The control circuit includes a plurality of gates in a flip-flop arrangement.
An electronic rotational sensor is disclosed in U.S. Pat. No. 3,845,375 of Ariel I. Stiebel. The sensor circuit utilizes a light sensing means which is pulsed. The pulses are varied by the rotating apparatus. Charging means in the form of capacitors produce a ramp voltage at a rate proportional to the rate of the apparatus rotation for controlling an SCR.
Emory E. Jones, III, in U.S. Pat. No. 4,025,832, shows an electronic slip detector circuit for use with a conveyor belt. The control includes a magnetic transducer operated by a plurality of magnets rotatable with a conveyor roller and means for converting the sensed pulses produced by the magnets to a DC voltage. When the voltage drops below an adjustable reference, the drive motor is de-energized.